The invention relates to a liquid coolant loop, in particular for an air conditioning installation for the passenger compartment of a vehicle.
JP-A-95 81383 describes such a loop comprising a compressor capable of raising the pressure of the coolant in the gaseous state, a condenser capable of condensing the coolant compressed by the compressor and of undercooling it to the liquid state, a preliminary pressure reduction device capable of lowering the pressure of the coolant coming out of the vessel and an evaporator capable of making the coolant coming from the pressure reducing valve pass from the liquid state to the gaseous state before its return to the compressor.
FIG. 1 is a diagram representing a thermodynamic cycle described by the liquid coolant in an air conditioning loop, traced in a system of enthalpy/pressure coordinates. In this system, a bell-shaped curve L envelops a zone of coexistence between liquid and gas, whereas the coolant is entirely in the liquid state to the left of the left-hand side of the curve and entirely in the gaseous state to the right of the right-hand side.
The cycle substantially has the shape of a rectangular trapezium with horizonal bases. From a point A situated in the gaseous zone, the compressor conveys the coolant in the gaseous state to a point B corresponding to a higher enthalpy and pressure than at point A. In the condenser, the coolant passes through a horizontal segment from point B to a point E situated in the liquid zone, which segment passes through the right-hand and left-hand sides of the curve L at points C and D respectively. Segments BC, CD and DE correspond respectively to a desuperheating of the gaseous coolant, to the condensation and an under-cooling of the coolant in the liquid state. At the entrance of the evaporator, the coolant is at a point G situated in the liquid/gas zone, corresponding to the same enthalpy value as point E and to the same pressure value as point A. In the evaporator, the coolant is returned to point A by passing through, at H, the right-hand side of the curve L.
In conventional liquid coolant loops, the coolant passes through the separating vessel at point E of the thermodynamic cycle, and passes through segment EG in the pressure reducing valve. As point E is situated in the liquid zone, the vessel is then completely filled with liquid and the quantity of coolant which it contains cannot vary. When the total mass of the liquid coolant contained in the loop falls, especially by virtue of leaks in the circuit, this reduction is performed in particular at the expense of the condenser, the undercooling capacity of which is thus reduced, which has the effect of raising the enthalpy level of the coolant at the outlet of the condenser and at the inlet of the evaporator and consequently of reducing the useful heat absorbed by the coolant in the evaporator.
One solution to this problem lies in departing from the conventional architecture by positioning the separating vessel between a condensation part and an undercooling part of the condenser, so that the thermodynamic state of the coolant in the vessel corresponds to point D of the cycle, situated on the saturation curve, which allows the vessel to contain a quantity of coolant that can vary as a function of the total mass of coolant in the circuit.
According to the invention, this same result may be obtained in a loop such as that defined in the introduction, by positioning between the condenser and the vessel a preliminary pressure reduction device which is capable of producing a pressure loss of between 1.5 and 14 bars so as to bring the pressure of the coolant back up to its saturated vapour pressure.
The preliminary pressure reduction device conveys the fluid from the thermodynamic state corresponding to the point E to that corresponding to the point F, situated again on the saturation curve, in which state the coolant continued in the separating vessel is consequently situated. The pressure reducing valve then conveys the coolant from point F to point G.
It is also known from JP-A-93223365 to place a device that produces a pressure loss between the condenser and the vessel. However, from this document it does not follow that this pressure loss brings the pressure of the coolant back again to its saturated vapour pressure. Furthermore, the only values disclosed for the loss of pressure are 0.5 and 1.0 kg/cm2, the latter value bringing about a substantial loss in cooling capacity of the loop.
Complementary or alternative optional characteristics of the invention are given below:
The preliminary pressure reduction device is capable of producing a pressure loss of between 4 and 10 bars.
The preliminary pressure reduction device comprises a constriction defining a minimum passage section of between 0.2 and 7 mm2 approximately in a pipe through which the entire flow of coolant leaving the condenser passes.
The passage section substantially retains its minimum value over a length of between 0.1 and 5 mm.
The minimum passage section does not exceed 50% of the passage section of the pipe upstream and/or downstream from the constriction.
The passage section reduces progressively in an initial region of the constriction, substantially retains its minimum value in an intermediate region and progressively increases in a final region.
The constriction is formed by an insert introduced into a substantially cylindrical pipe.
The constriction is formed by a thickening of the wall of a substantially cylindrical pipe.
The constricted passage is adjacent to the cylindrical wall.
The constricted passage is substantially centred in relation to the cylindrical wall.
The constriction is formed by a washer lock-beaded into the pipe.
The preliminary pressure reduction device is housed in an outlet pipe mounted on a manifold of the condenser.
The said outlet pipe is contained in the separating vessel and discharges into a gas collection space thereof.